1) Field of the Invention
The present invention relates to the coating of a workpiece and, more particularly, to the coating of a portion of a workpiece with aluminum to provide corrosion resistance.
2) Description of Related Art
Friction welding is a known process for forming joints and/or changing the grain structure in a workpiece. For example, friction stir welding is a process in which a rotating tool, such as a pin or probe, is urged into and/or through a workpiece, e.g., to join multiple structural members of the workpiece in a solid state or to repair cracks in a workpiece. As shown in FIG. 1, the probe 12 of the friction stir welding tool 10 typically extends from a shoulder 14, which can be flat, concave, or otherwise contoured, and the shoulder 14 is urged against the workpiece 20 so that the probe 12 is urged into the workpiece 20 at the interface 22 of the abutting structural members 24, 26 that make up the workpiece 20. The probe 12 is then urged along the interface 22 to form a continuous weld joint 28. For example, according to one conventional friction stir welding process for forming a butt joint, a friction stir welding machine 30, which controls the rotation and movement of the tool 10, urges the probe 12 in a direction 16 and into the interface 22 between the two structural members 24, 26. The rotating tool 10 is then moved in a direction 18 along the interface 22. The motion of the rotating tool 10 generates frictional and deformation heating, thereby forming a region of plasticized material in the structural members 24, 26. In some cases, the tool 10 is tilted, such as approximately 2.5°, relative to the structural members 24, 26 so that the trailing edge of the shoulder 14 is thrust into the plasticized material. Upon cooling of the plasticized material, the members 24, 26 of the workpiece 20 are joined along the weld joint 28. Friction stir welding is further described in U.S. Pat. No. 5,460,317 to Thomas et al., the entire contents of which are incorporated herein by reference.
Friction stir welding has been demonstrated to be a successful joining method for forming certain types of joints, such as the butt joint shown in FIG. 1 or a lap joint in which the probe is inserted in a direction substantially perpendicular to the interface between overlapping structural members. In addition, other types of friction welding have also been shown to be useful for forming certain joints. However, in some cases, typical friction welding can result in a joint that exhibits a corrosion resistance that is less than desired. In particular, in some cases, the friction welded joint of the workpiece where the material has been mixed during friction welding can be more susceptible to corrosion than the areas of the workpiece remote from the joint. In addition, the heat associated with friction welding typically results in a heat affected zone proximate to the joint, where the heat of friction welding has affected the material properties of the workpiece. The heat affected zone can also be susceptible to corrosion. Alternatively, even if the joint and the heat affected zone of the workpiece are not more susceptible to corrosion than other parts of the workpiece, it may be desirable to increase the corrosion resistance of those portions of the workpiece. In some cases, the corrosion resistance of the friction weld joint, the heat affected zone, and/or other portions of the workpiece can be improved by thermally processing the workpiece. However, such thermal treatments generally increase the time and/or expense for manufacturing the workpiece.
Similarly, the corrosion resistance of other type of welds can be characterized by a corrosion resistance that is less than the desired corrosion resistance. Other welds include those formed by other types of solid state welding, arc welding, resistance welding, and the like.
Thus, there exists a need for an improved method for increasing the corrosion resistance of a weld joint, including joints formed by friction welding and other types of welding, and an associated workpiece having such an improved corrosion resistance. The method should be compatible with different configurations of welds.